1. Field of the Invention
This invention relates to a furnace. It relates also to a method of carrying out a reaction, utilizing the furnace.
2. Summary of the Invention
According to a first aspect of the invention, there is provided a furnace comprising a furnace shell rotatable about a rotational axis, the furnace shell providing a furnace chamber for holding a solid particulate reagent as the furnace shell rotates; and at least two electrodes exposed to the chamber and being mounted in electrically insulated fashion therein, with the electrodes being spaced apart so that solid particulate reagent in the furnace chamber can be heated up by direct resistance heating thereof, utilizing the electrodes.
The furnace shell will normally be cylindrical, and be located substantially horizontally so that the rotational axis extends substantially horizontally.
The furnace is suitable for carrying out reactions whereby a solid particulate reagent is reacted with a further reagent at elevated temperature. The reactions may be endothermic or exothermic. Examples of endothermic reactions are nitriding of a titanium-containing material as the solid particulate reagent, such as a titaniferous ore or slag, to convert titanium values therein to titanium nitride; nitriding of a vanadium containing material; and the carbiding of silicon-bearing material. The solid reagent is thus heated to said elevated temperature, and is supplied with heat for the endothermic reaction at said elevated temperature, by direct resistance heating thereof in the furnace chamber. The furnace can also be used for the regeneration of spent activated carbon.
In other words, an electrical potential or voltage is applied to the material of the solid reagent whereby an electrical current is passed by means of the electrodes, through the material, thereby generating heat within the material, to raise the temperature of the material to the elevated temperature, to supply heat at that temperature for the endothermic reaction (where applicable) and/or to enhance the reactivity of the reactants. Thus, the electrodes will be of different polarity.
Thus, according to a second aspect of the invention, there is provided a method of carrying out a reaction which includes heating the solid reagent to the elevated temperature in the furnace chamber of a furnace as hereinbefore described.
The method may include introducing a second or further reagent into the chamber, to react with the solid reagent. The further reagent may be a further solid reagent, but is typically a gaseous reagent. The gaseous reagent can then be passed over the solid reagent in the furnace chamber and/or through it, e.g. by being introduced into the bottom of a bed of the material in the chamber.
By `particulate` is meant any desired particle shape and size. Thus, the solid reagent particles can have an irregular shape and fall within a predetermined range of sizes, as would be the case when it comprises ore which has been mined and milled. Instead, it can be of regular shape and size, e.g. in the form of a powder, granules, pellets, briquettes, or the like.
The elevated temperature at which the reaction takes place may be 1000.degree.-1800.degree. C., preferably 1100.degree.-1600.degree. C. and more preferably 1200.degree.-1350.degree. C., and the voltage applied to the material will be selected accordingly, bearing in mind the resistivity of the material.
Certain solid reagent materials to which it is contemplated the method will in practice be applied, such as titaniferous ores (e.g. ilmenites) or titaniferous slags, which are to be reacted with gaseous nitrogen to nitride titanium values therein, can be relatively non-conductive to electricity at ambient temperatures. For such materials, initial heating may be by other methods, such as preheating the solid reagent by radiative, convectional and/or thermal-conductive methods, to raise the temperature of the material from ambient temperature up to an intermediate value at which ohmic- or direct-resistance heating is effective, after which the ohmic- or direct-resistance heating can be employed further to raise the temperature of the material up to its final value, and to supply the heat needed for the endothermic reaction. Such intermediate value may be 700.degree.-1300.degree. C., preferably 700.degree.-1000.degree. C., e.g. 700.degree.-800.degree. C.
Instead or in addition, the method may include the step of mixing the solid reagent in particulate form with a particulate solid electrical conductor, to provide a mixture having increased electrical conductivity compared with that of the solid reagent. The mixture may be compacted or consolidated, e.g. by pelletising, extruding or briquetting the mixture, further to increase said conductivity. For the mixing, the solid reagent and electrical conductor may be in finely divided form, having a particle size of at most 1000 .mu.m, e.g. 50-200 .mu.m, and the consolidation, at least in the case of briquetting, may be by subjecting the mixture to a pressure of at least 8-11 MPa. Carbon may be employed as the electrical conductor, and has the advantage, in the nitriding of titanium-containing solid reagents, of providing a reducing environment for the endothermic nitriding reaction. The pellets or briquettes may be in the size range 5-80 mm, e.g. 10-20 mm.
When carbon is used as the electrical conductor, it may form 10-90% by mass of the mixture, e.g. 12-60% thereof. The carbon may be in the form of coal, anthracite, coke, industrial char, charcoal, graphite or the like, in particular duff coal, which is readily obtainable and inexpensive.
The ohmic- or direct-resistance heating is thus applied to a moving bed of the solid reagent, e.g. a moving bed of said pellets or briquettes, in the furnace chamber as it rotates so that any preferential paths through the bed of material along which electrical currents pass in response to the applied voltage are continuously or intermittently disrupted, and so that more or less uniform heating of the particulate material is promoted.
The spacing between the electrodes may be 100-1000 mm, typically selected on the basis of the loading or proposed loading of solid material in the furnace chamber, ie the furnace chamber capacity, the resistivity of the solid material, and the required operating voltage. Such spacings permit operating voltages conveniently of 100-200 V to be used, although higher spacings of up to 1,5-3 m or more, requiring voltages of 350-500 V or more, can in principle be feasible.
The power supply used may be AC or DC.
In accordance with the method, the operating voltage between the electrodes may be altered from time to time, either manually or automatically by means of an automated control system, which may be electronic, in response to changes in the temperature of the solid reactant in the interior of the furnace, ie in the furnace chamber, which temperature may be sensed e.g. by suitably located thermocouples in the interior of the furnace. In this way, the operating voltage can be increased to increase the power supply to the furnace and hence to increase the temperature of the solid reagent, or said voltage can be reduced to reduce the power supply and temperature. In one embodiment three voltages may be employed, e.g. 60 V, 110 V and 220 V, the lowermost voltage being used when the solid reagent temperature exceeds a desired value by more than a predetermined amount, the uppermost voltage being used when said reagent temperature falls short of the desired value by more than a predetermined amount, and the intermediate voltage being used when the reagent temperature is closer to the desired value than said predetermined amounts. Instead, e.g. 380 V can be used for start-up, whereafter two voltages such as 110 V and 200 V may be used, the lower voltage being used when the reagent temperature is above the desired temperature and the higher voltage being used when the reagent temperature is below the desired temperature.
When the particulate solid electrical conductor which is mixed with the solid reagent is carbonaceous, e.g. duff coal, the heating of the solid reagent to operating temperature can give rise to the production of a combustible off-gas in the interior of the furnace, containing carbon monoxide, vaporized volatile coal constituents or the like. The method may include the step of burning this off gas to provide the heat used for preheating the solid reagent, as described above, although electrical or any other suitable heating may naturally be used instead.
When the furnace is operated with a nitrogen atmosphere, as mentioned above, and a carbonaceous particulate solid electrical conductor is used, mixed with a solid reagent containing titanium values, suitable control of the reaction environment in the furnace can permit not only the nitriding of the titanium values, but, instead or in addition, the carhiding, carbonitriding or oxycarbonitriding thereof, which permits the production, as desired, of titanium nitride, titanium carbide and/or titanium carbonitride. It will further be appreciated that, although the description of the present invention emphasizes the nitriding of titanium, it may easily, in analogous fashion, be applied to reactions involving other solid reagents, e.g. for the nitriding, carbiding or carbonitriding thereof, such as in the production silicon carbide by reacting a solid reagent comprising silicon with a solid carbon-containing reductant in an inert environment in the furnace. Bearing in mind that the carbonaceous particulate material such as duff coal can have the functions, for a titanium-containing solid reagent, of both increasing electrical conductivity of the solid reagent and of providing off-gas for preheating, an excess thereof is preferably used over the stoichiometric requirement for reducing all the titanium (as the oxide) in the solid reagent, conveniently double said stoichiometric requirement of carbonaceous material is used.
Surprisingly, the Applicant has found that, in the case where the solid reagent is vanadium-bearing material, titanium-bearing material, or silicon-bearing material, the increased conductivity of the solid reagent (whether or not it is with any particulate conductor to raise its conductivity) achieved by preheating the solid reagent, is related to the rate of heating the solid reagent, and is related to the rate at which any carbonaceous material mixed with the solid reagent is devolatalized. It is accordingly desirable to preheat as quickly as possible, e.g. at least 20.degree. C./min, preferably at least 80.degree. C./min.
The method may be carried out batchwise, whereby a charge of solid reagent is charged into the furnace chamber and heated to cause the required reaction to take place, before being discharged and replaced by a succeeding charge; or it may be continuous, a stream of solid reagent passing continuously through the furnace, where it is subjected to required reaction.
The furnace may thus be constructed to cause or permit passage therethrough of both the solid reagent and the gaseous reagent, to permit the continuous operation, and may have an interior which is sealed off from the atmosphere. The furnace shell may comprise an outer skin or wall, lined with a suitably inert shock-resistant electronically non-conductive and thermally insulating refractory lining, e.g. a calcium silicate and/or an .alpha.-alumina lining; and the spacing of the electrodes, which may be of copper, silicon carbide or preferably of graphite, may be as described above. The electrode material will be selected according to the operating temperatures and conditions. Thus, at lower operating temperatures, copper electrodes can be used, while at higher temperatures, graphite electrodes can be used. While the furnace may in principle have any suitable construction, such as a vibratory table located in its furnace chamber, to convey the solid reagent through its interior, e.g. from an inlet to its furnace chamber to an outlet therefrom for continuous operation, the furnace is conveniently such that rotation of the furnace shell causes passage of solid reagent through or along its chamber. The furnace will naturally include suitable drive means for driving the shell to rotate.
The furnace or kiln may be provided with an alternating current (AC) or direct current (DC) power supply to the electrodes, via one or more suitable slip-rings mounted on the furnace. Similarly, the furnace may have a slip-ring arrangement connected e.g. to one or more thermocouples arranged in the furnace chamber, for monitoring the temperature in the chamber. The electrodes may thus be arranged in one or more pairs in the interior of the furnace so that they are located at suitable locations and spacings whereby the passage of an electrical current between the electrodes of each pair in response to application thereto of a sufficient electrical potential is promoted, and the passage of electrical current between electrodes of different pairs is discouraged.
The potential difference between the electrodes of a pair, measured through the solid reagent, is proportional to the distance between the electrodes, so that the distance between the electrodes of a pair is in principle limited only by the voltage supply available; however, the voltage is also a function of the nature and resistivity of the solid reagent. Excessive voltages can cause difficulties related to unwanted electrical discharges between the electrodes across the surface of the solid reagent, along the surface of the insulating refractory lining of the furnace or through the refractory lining to the exterior of the furnace.
According to one embodiment of the invention, each of the electrodes may be of annular form and extend circumferentially along an inner surface of the furnace shell while protruding radially inwardly therefrom, with the electrodes being spaced axially or longitudinally apart.
However, the invention also contemplates the provision of a plurality of pairs of the electrodes in the furnace, the electrodes of each pair being spaced from one another and the pairs of electrodes being arranged and located in the interior of the furnace so than electrical discharges will take place only between the electrodes of said pairs, and not between electrodes of different pairs; and so that a relatively long rotary furnace can be used with relatively small spacings between the electrodes of each pair, thereby permitting relatively low voltages (e.g. 100-250 V) to be used. Thus, for example, the pairs of electrodes may be spaced longitudinally from one another.
According to another embodiment of the invention, one of the electrodes (`the first electrode`) may extend centrally along the rotational axis, with a plurality of the other electrodes (`the second electrodes`) being provided, the second electrodes protruding from and extending along an inner surface of the furnace shell in a longitudinal direction, and being spaced circumferentially from one another. The central electrode will thus be of a particular polarity, with the second electrodes being of opposite polarity, to provide for current flows between the central electrode and those second peripheral electrodes which are at any time submerged by the particulate material in the furnace, the furnace being operated with a bed of particulate material therein of sufficient depth to be in contact with the central electrode.
In yet a further embodiment, a plurality of the electrodes, arranged in pairs, e.g. three pairs, and protruding from and extending along an inner surface of the furnace shell in the longitudinal direction, may be provided, with the pairs being circumferentially spaced from one another, and the electrodes of each pair being spaced circumferentially from each other by a spacing which is less than the spacing between adjacent pairs. In this case, as with the electrodes discussed above, the electrodes may stand proud of the surface of the lining. They can then also act as lifters for lifting particulate material in the kiln as it rotates, thereby assisting in keeping the particulate material continuously in motion and mixing it, to disrupt the paths of electrical currents flowing therethrough. In a still further embodiment, the electrodes may be of non-annular form, and protrude from an inner surface of the furnace shell, with the one electrode (`the first electrode`) being spaced longitudinally from the other electrode (`the second electrode`). A plurality of the first electrodes, circumferentially aligned and spaced apart circumferentially, and being of the same polarity, as well as a plurality of the second electrodes, circumferentially aligned and spaced apart circumferentially, and being of the same polarity, may be provided. Thus, the first electrodes will be in the form of a group, while the second electrodes will also be in the form of a group, with the groups being spaced axially or longitudinally and the electrodes of one group being of different polarity to those in the other groups. The electrodes of the first group may be aligned with those in the second group, in the longitudinal direction. If desired, a further group of the first electrodes, spaced axially or longitudinally from the group of second electrodes so that the group of second electrodes is located between the two groups of first electrodes, may be provided.
The Applicant has found, that in certain cases, the resistivity of the solid reagent decreases as the temperature of the solid reagent increases with heating thereof and/or as the solid reagent reacts progressively with the gaseous reagent (thereby progressively changing the composition of the solid reagent) so that, after such heating and/or reaction, a relatively lower voltage is required to maintain a consistent current flow in the solid reagent.
Thus the method may include the step of passing the solid reagent through a series of successive furnace chambers or reaction zones, each chamber or zone including said at least two electrodes. The spacing between the electrodes in each succeeding zone may then be greater than that of the preceding zone. The method may in this case, in particular, include passing the solid reagent through the zones so that it forms a separate bed in each zone, with the beds being electrically isolated from each other.
Thus, the material dams up in each segment, so that at least some parameters can be controlled separately in each segment, e.g. temperature, residence time, and applied voltage.
In this way, by selecting zones of appropriate size or length-for a particular solid reagent and a particular gaseous reagent, substantially the same voltage may be used for each pair of electrodes in each of the zones, despite variations of the spacing between the electrodes in the different zones. In particular, in the case of three zones, a single three-phase source of power can be used with one said phase supplying power to each of said zones.
Thus, the furnace may include a second substantially horizontal cylindrical furnace shell rotatable about the rotational axis and spaced axially or longitudinally from the other or first furnace shell, the second furnace shell providing a second furnace chamber or reaction zone which is in communication with the first furnace chamber or reaction zone and through which solid particulate reactant from the first chamber can pass in the longitudinal direction, and with said at least two electrodes also being provided in the second furnace chamber. If desired, at least one further similar horizontal cylindrical furnace shell may be provided adjacent the second furnace shell, to provide the successive reaction zones.
The first and second chambers may have the same or different diameters. For example, the second chamber may have a greater diameter than the first chamber.
Additionally or instead, the first and second furnace shells may be of the same or different length, and the spacing between the electrodes of the first furnace shell may be the same of different to that of the electrodes of the second furnace shell.
In other words, the furnace can thus be segmented, comprising a series of axially spaced portions or segments, each containing a pair of the electrodes. Each segment may be of a different diameter from the adjacent portion or segment. Thus, the furnace may comprise a plurality of such segments increasing progressively in diameter from one portion to the next, the portion of smallest diameter being at the upstream end of the furnace relative to the direction of solid reagent flow.
Instead, the furnace may comprise a plurality of successive segments of the same or generally similar diameter, each successive segment being longer, in a downstream direction, than the segment preceding it, and the distance between the electrodes of each successive segment being correspondingly greater. This construction takes advantage of the finding by the Applicant, referred to above, that the resistivity of the solid reagent, and hence the voltage required to cause passage of a current of a given value through a given mass or volume of the solid reagent, decreases as the temperature of the solid reagent is increased and/or as the solid reagent progressively undergoes reaction with the gaseous reagent.
For example, the furnace may comprise three successive segments, each having first and second electrodes, in which the distance between the electrodes in successive segments is 650-750 mm, 850-950 mm and 1050-1250 mm respectively, the inner diameter of the furnace chamber being 500 mm.
The inner surface of the lining of the furnace is preferably smooth and both non-porous and electrically insulating, so that impregnation thereof or coating thereof by solid reagent and particularly by any particulate solid conductor added to said reagent is discouraged. As mentioned above, .alpha.-alumina such as castable .alpha.-alumina, has been found to be suitable for this purpose.
Preferably the furnace has its interior closed off from the atmosphere and/or is operable at above atmospheric pressure to permit maintenance of a controlled atmosphere therein.
The furnace axis may be tilted at an angle of about 1.degree.-3.degree., preferably about 2.degree. to the horizontal, the downstream end being the lower end, to assist in passage of solid material through the chamber.
The furnace may be provided with longitudinally spaced annular isolating partitions for electrically isolating solid reagent in one segment from that in an adjacent segment. The partitions will be of a refractory and preferably insulating material. The furnace may, further, be provided with lifting members or bars which, as the furnace rotates, cause solid material to be lifted and transferred progressively from one segment to an adjacent segment.
The furnace may include feed and extraction means for feeding and extracting solid reagent and waste product therefrom, as well as gas feed means and gas extraction means for feeding gas into and withdrawing it from the chamber respectively.
The gas feed means may include a plurality of gas permeable distributors in the furnace shell, the distributors being circumferentially spaced from one another; gas delivery means for delivering gas to the outsides of the distributors, with the gas passing through the distributors into the furnace chamber; and gas flow control means operable, during rotation of the furnace, to deliver gas only to those distributors which are at or near their lowermost position so that, in use, inflowing gas passes largely through a charge of solid particulate reagent in the furnace chamber.